Hi, I have partially disassembled the Sea&Sea YS110 and YS50 strobes, they should be similar to the S&S YS-01. Here are the tools and steps I used.
Volt meter,
Small philips screwdriver,
Razor blade,
Wall lamp with 110V or 220V bulb and electric plug, connect plug terminals to test wires with insulated alligator clamps, or use an insulated probe to discharge the capacitors.
Camera.
Pencil and note book
Take photos and notes,
Jar with cap for parts removed parts.
Store all removed parts in jar.
Steps:
First remove batteries.
Remove the three tabs on the sides of strobe with the razor.
Unscrew the three small philips screws.
Pull the front end from the back.
Remove and inspect main O ring.
Once the strobe is open, with a DVM test each capacitor to see if it is charged. The charge can be up to 350 V.
If capacitor is charged, I short it with an insulated screwdriver, the discharge spark sounds like a pistol fired indoors.
Or better, discharge by connecting the capacitor terminals to the lamp plug terminals, the lamp's bulb should glow for a few seconds as the capacitor discharges. Measure voltage again till it is zero.
You mention your strobe's on/off function: rotary switch or on/off circuit, misbehaves, it is deep inside the upper strobe half to one side of the battery compartment, and I have not disassembled this part of the strobe.
Please tell us of your progress.